1. Field of the Invention
This invention relates to a supersonic or ultrasonic wave flaw detecting apparatus for detecting acoustic boundaries of flaws and/or abnormal parts of materials within an object to be inspected, by the use of supersonic or ultrasonic waves and for displaying the position and shape of the flaws or abnormality.
2. Description of the Prior Art
A well-known method for detecting a flaw and the like within an object by the use of supersonic waves includes what is called the pulse echo method. In this method, supersonic wave pulses are made to penetrate the object to be inspected and the absence or presence of a flaw is determined by receipt of an echo signal reflected and returned from the inside of the object.
It is also well known that the most widely used method for displaying the signal derived in the pulse echo method is the A scope method in which the chronological changes in signal intensity, namely, a signal waveform is displayed. The A scope method, however, has the disadvantage that it is often difficult to determine at which points within the object the many echo signals displayed on the display screen of the cathode ray tube are reflected. This is especially difficult when a complicated shape of the object is involved.
Beside the above-mentioned A scope method having such a disadvantage, there is what is called the B scope method. In this method, the scanning lines on CRT is brightness-modulated by the echo signals and displayed in synchronism with the movement of penetration path of the supersonic wave beam thereby to produce a sectional image of the object. The advantage of this method is that the image due to the echo signals based on the presence of a flaw or abnormality of material represents the relative position of the abnormality within the section of the object as well as the shape thereof, thus greatly facilitating the checking of the inspection results.
When the object to be inspected is made up of a living body, its internal organ or frame-work forms acoustic boundaries. In the event that the boundary surfaces within the object on which supersonic waves are reflected are in various directions, what is called the compound scanning system is employed. In this method, the scanning by sectorially changing the direction of the path of the incident supersonic wave beam is combined with the scanning by moving the point of transmission and receipt of the wave, namely, the position of the probe. The scanning lines on the display section comprising a CRT is relocated in synchronism with the path of the supersonic wave beam which is changing at every moment. The scanning lines are brightness-modulated by echo signals received, thereby displaying the state of a sectional area of the object to be inspected.
Specifically, the sector scanning according to the compound scanning system employs two different methods. One of them is the water immersion method, in which the direction of the probe is capable of being changed freely by using such a liquid as water as a transmission medium for supersonic waves. The other method is one used for an object such as a living body having soft boundary surfaces, in which the direction of the probe, namely, the direction of transmission of the supersonic wave beam is changed while maintaining the probe in direct contact with the object to be inspected.
The last-mentioned direct-contact method cannot be used when the object to be inspected is made of metal or other hard material and in addition has a sound speed different from that in the transmission medium of the supersonic wave.
In the case of the water immersion method using water or the like as a contact medium, the transmission path of the supersonic wave beam is refracted at the plane of incidence of wave at the surface of the object of inspection. Therefore, it is necessary to refract the scanning lines for indication of images on the B scope in accordance with the transmission path. In the event that the length of the transmission path of the supersonic wave in a liquid from the transmission point of the supersonic wave pulses to the point of incidence into the object of inspection undergoes variations, it is quite difficult and requires very high skill to refract the scanning line in the above-mentioned manner.
The display scanning lines on the screen of the conventional display apparatus is generated by the sweep in accordance with the speed and direction of transmission of supersonic wave in the object of inspection. In such a conventional apparatus, the speed and direction of sweep is determined on the basis of only one of the two modes; one is a longitudinal wave (in which the direction of displacement of the transmission medium coincides with the direction of propagation of the wave) and the other is a shear wave (in which the direction of displacement of transmission medium is perpendicular to the direction of propagation of the wave). In the case where the incident angle of the supersonic wave is in the vicinity of zero, namely, in the case where the supersonic wave enters the object at substantially right angles to the surface thereof, the supersonic wave penetrating the object mainly follows the mode of longitudinal wave. Accordingly as the incident angle is increased so that the supersonic wave enters the object obliquely, however, the influence of the shear wave is gradually increased, and the modes of both longitudinal and shear waves come to coexist in the object of inspection. As a result, with the increase in the incident angle, the shear wave mode undesirably exists in addition to the required longitudinal wave mode. Thus a useless and harmful image as well as the image of a section of the object originally intended for is displayed. This often leads to an error in examination of the results of supersonic wave flaw-detecting operation and forms a roadblock to successful flaw detection.
With further increase in incident angle, the efficiency of incidence of the longitudinal wave into the object is sharply decreased, thus posing another problem of reduced sensitivity of flaw detection by the longitudinal wave.